1. Technology Field
The present invention generally relates to x-ray generating devices. In particular, the present invention relates to a system that prevents leakage of liquid from an apparatus, such as an x-ray tube, that utilizes a diaphragm.
2. The Related Technology
X-ray producing devices, such as x-ray tubes, are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray tubes operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of the x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a reservoir for a cooling liquid, such as dielectric oil, that serves both to cool the x-ray tube and to provide electrical isolation between the tube and the outer housing.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted as a beam through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam is then directed toward an x-ray subject, such as a medical patient, so as to produce an x-ray image.
Generally, only a small portion of the energy carried by the electrons striking the target surface of the anode is converted to x-rays. The majority of the energy is rather released as heat. It is critical to remove excess heat produced during x-ray production to prevent failure of the x-ray tube. One common method in dissipating heat involves submerging the evacuated enclosure in a dielectric cooling liquid which, as explained above, is contained within a reservoir defined by the outer housing. The cooling liquid assists in absorbing heat from the evacuated enclosure that is produced therein during x-ray production and dissipating it to the surrounding environment. Such dissipation can be accomplished, for example, via conductive heat transfer between the cooling liquid and the surface of the outer housing. In this way, the operating temperature of the x-ray tube is maintained within acceptable levels.
In many liquid-filled x-ray tubes, one or more diaphragms are employed in order to maintain a relatively consistent liquid pressure within the reservoir at or near atmospheric pressure (“1 atm”). These diaphragms are flexible and many include an interior surface in liquid communication with a portion of the cooling liquid, and an exterior surface, which is in communication with the tube exterior such that it is subject to atmospheric pressure. During tube operation, heat created as a result of x-ray production is absorbed by the cooling liquid. Absorption of this heat causes the volume of the cooling liquid to expand. In response to this volume expansion, the diaphragm contracts, thereby expanding the relative size of the reservoir, which reduces the pressure of the cooling liquid.
Similarly, when cooling of the liquid occurs, its volume and corresponding pressure decrease. Expansion of the diaphragm is then triggered, which reduces the liquid reservoir volume, thereby increasing cooling liquid pressure. The diaphragm is configured and operated in this manner to maintain the cooling liquid pressure at or near 1 atm during tube operation, notwithstanding the cyclical temperature changes of the cooling liquid. This in turn enables the fluid-tight seals of the x-ray tube outer housing to be configured for mere liquid containment, and not for liquid containment at elevated pressures relative to atmospheric pressure. This consequently reduces both the complexity and cost of x-ray tube seals, thereby offering added savings for tube manufacturing.
Despite their utility in maintaining constant cooling liquid pressure, several challenges nevertheless exist with respect to diaphragm use. Many of these challenges relate to the unintended rupture or other failure of the diaphragm. When such failure occurs, escape of cooling liquid past the diaphragm can result. Further, because many tube designs require that the diaphragm be exposed to atmospheric pressure and therefore lack a fluid-tight seal about the diaphragm, cooling liquid that escapes past the diaphragm can also spill from the x-ray tube entirely. Such spillage is highly undesirable. As can be imagined, liquid escape from the x-ray tube not only presents a contamination problem, but can create a hazardous situation, presenting a health risk to tube users, patients, or others in close proximity to the x-ray tube. In particular, x-ray tubes are often employed in connection with medical x-ray scanning devices, such as CT scanners. An x-ray tube utilized in CT scanners are often mounted on a rotating gantry that achieves high rotational rates during scanning operations. Should the diaphragm of a CT scanner x-ray tube so positioned fail during use, extensive cooling liquid leakage and dispersal from the tube can result, including exposure to the local environment, users, patients, etc. As described above, cooling liquid often possesses significant quantities of absorbed heat, as described above, which can present a burn risk to those exposed to the liquid. Furthermore, some cooling liquids are hazardous substances and create an undesired contamination risk. For these and other reasons, diaphragm failure and its attendant consequences are to be avoided.
In an effort to reduce the. effects of diaphragm failure, some known x-ray tubes hermetically seal the diaphragm off within the outer housing and isolate it from atmospheric pressure influences. Though this alleviates liquid containment problems should the diaphragm fail, it nevertheless represents a significant additional expense in manufacturing such tubes, as all fluid-tight seals used in the outer housing must be designed to withstand the elevated pressure that result from such a tube design.
Another attempt at avoiding the above challenges has involved tubes that employ a dual diaphragm system, wherein a first diaphragm is backed by a backup second diaphragm in the outer housing of the x-ray tube. Though this dual diaphragm design can in certain cases enhance the safety of the x-ray tube in the event of a single diaphragm failure, both diaphragms must still be subject to atmospheric pressure, and therefore are still susceptible to the above undesirable consequences should failure of both diaphragms occur. Further, a dual diaphragm system is necessarily more complex than a single diaphragm system, thereby equaling greater production costs and more complication when tube servicing is required, as well as creating more possible failure points, given the extreme operating conditions in which x-ray tubes are often utilized.
In light of the above, a need exists for an x-ray tube having a diaphragm system that avoids the above problems. In particular, an x-ray tube having a sealing system that protects from cooling liquid escape in the event of diaphragm failure is needed. Such a solution should be easily adaptable to the variety of x-ray tube types and other apparatus without substantially increasing the complexity thereof. Any solution should also be adaptable to multiple diaphragm configurations found in these apparatus. In addition, any solution should not interfere with the operation of the diaphragm in maintaining a constant cooling liquid pressure within the x-ray tube.